Techniques that use sharp tips, with apex dimensions on the nanometer-scale, for imaging and investigating the structure of materials down to the atomic scale, are known. Such techniques include atomic force microscopy (AFM) and scanning tunneling microscopy (STM), as disclosed in EP 0 223 918 B1 and U.S. Pat. No. 4,343,993.
Based on the developments in scanning tunneling microscopy and atomic force microscopy, new storage concepts have been introduced over the past few years that profit from these technologies. Probes having a nanoscale-sized tip have been introduced for modifying the topography and for scanning an appropriate storage medium. Data are written as sequences of bits represented by topographical marks, such as indentation marks and non-indentation marks. The tips comprise apexes with a radius in the lower nanometer range and the indentation marks have, for example, a diameter in the range of 20 to 40 nm. Hence, these data storage concepts promise ultra-high storage area density.
A storage device for storing data based on the AFM principle is disclosed in “The millipede—more than 1,000 tips for future AFM data storage” by P. Vettiger et al., IBM Journal Research Development, Vol. 44, No. 3, March 2000. The storage device has a read and write function based on a mechanical x-, y-scanning of a storage medium with an array of probes each having a tip. The probes operate in parallel, each probe scanning, during operation, an associated field of the storage medium. In this way, high data rates may be achieved. The storage medium may comprise a thin polymethylmethacrylate (PMMA) layer. The tips are moved across the surface of the polymer layer in a contact mode. The contact mode is achieved by applying small forces to the probes so that the tips of the probes can touch the surface of the storage medium. For that purpose, the probes comprise cantilevers carrying sharp tips on their end sections. Bits are represented by indentation marks or non-indentation marks in the polymer layer. The cantilevers respond to these topographic changes in the surface while they are moved across the surface. Indentation marks are formed on the polymer surface by thermomechanical recording. This is achieved by heating a respective probe with a current or voltage pulse during the contact mode in a way that the polymer layer is softened locally where the tip touches the polymer layer. The result is a small indentation on the layer having a nanoscale diameter.
Reading is also accomplished by a thermomechanical concept. The heater cantilever is supplied with an amount of electrical energy, which causes the probe to heat up to a temperature that is not high enough to soften the polymer layer as is necessary for writing. The thermal sensing is based on the fact that the thermal conductance between the probe and the storage medium, especially a substrate on the storage medium, changes when the probe is moving in an indentation as the heat transport is in this case more efficient. As a consequence of this, the temperature of the cantilever decreases and hence, also its electrical resistance changes. This change of electrical resistance is then measured and serves as the measuring signal.
WO 02/061734 discloses a data storage device with a storage medium in the form of a polymer layer and a cantilever comprising a tip with a nanoscale apex being located in close contact with the storage medium. The data storage device is designed to write data to the storage medium by a combination of applying a local force to the polymer layer via the tip and applying energy to the surface in the form of heat via the tip by passing a write current through the cantilever. For this reason, the cantilever comprises a heater element to be heated up. Data are represented by indentation marks or non-indentation marks in the storage medium. For selectively erasing indentation marks, new pits are formed overlapping each other over pre-recorded indentation marks to be erased. In this way, the surface of the storage medium is substantially leveled. This is achieved by performing the write operation, that is by heating up the tip to a level sufficient to locally deform the polymer layer. This write operation is, for the purpose of erasing, performed with a high density of new pits overlapping each other so that each new pit effectively erases the immediately preceding pit. WO 02/061734 further discloses that, in the erase operation, the force being applied from the tip may be reduced or removed altogether. The application of the heating energy excites molecules in the surface of the storage medium and intermolecular forces in the surface of the storage medium are then sufficient to push the tip out as the surface of the storage medium relaxes into its stable state.